SIBLING REVELRY: Ted, John and Bobby Kennedy held sex parties with Marilyn Monroe and Rat Packers in JFK's suite at The Carlyle, according to an FBI report prompted by a tip from a mob stoolie.Popperfoto/Getty Images

The FBI was tipped off that the three Kennedy brothers, Marilyn Monroe and members of Frank Sinatra’s Rat Pack held sex parties in John F. Kennedy’s suite at The Carlyle hotel, according to confidential bureau files revealed yesterday.

An FBI report on the alleged orgies at the Upper East Side hotel was prompted by a July 1965 disclosure from a “reliable” Mafia informant that the mob wanted to use women supplied by “associates of Frank Sinatra” to embarrass the Kennedys in New York.

The women were to “be placed in compromising situations” with Robert and Ted Kennedy and their brother-in-law, actor Peter Lawford, the confidential memo said.

The mob wanted to smear Robert F. Kennedy because of his war on organized crime as US attorney general from 1961 to 1964, the informant said. But the FBI didn’t investigate the claims, the bureau said.

The documents were among 2,352 pages of FBI files unveiled yesterday on Ted Kennedy, who died last August after a battle with cancer.

The bulk of the material detailed dozens of death threats made against Kennedy over two decades. But among the other disclosures:

* The file on the mob plot began with the informant’s disclosures to the FBI’s Milwaukee office. The FBI added an unsigned statement that said a multimillionaire Manhattan divorcée knew about the orgies.

Among those who took part were John, Robert and Ted Kennedy, Monroe, Sinatra, Sammy Davis Jr. and Lawford and his wife, Patricia Kennedy, it said.

The statement indicated Hammond, who was divorced from a US ambassador, was credible.

It was widely reported in the early 1960s that John Kennedy kept a two-bedroom apartment at The Carlyle, and it was later claimed that he spent the night there with Monroe after she sang “Happy Birthday, Mr. President” at his 45th-birthday celebration.

An FBI summary of the documents released yesterday said the bureau didn’t consider the Milwaukee and Hammond information “solid.”

* In 1977, the FBI investigated a bizarre claim from a California prison inmate who had spent 18 months in a cell next to Sirhan Sirhan, RFK’s assassin.

The unidentified inmate told agents that Sirhan offered him $1 million and a car in return for killing Ted Kennedy when he got out of jail. The inmate said he refused the offer.

* The FBI was notified almost immediately after the July 19, 1969, car accident on Chappaquiddick Island that killed secretary Mary Jo Kopechne. But police told the bureau that the fact that Kennedy was the driver “is not being revealed to anyone.”

* A cryptic Oct. 17, 1969, note from Cartha DeLoach, the FBI’s No. 3 man, said Nixon administration official John Dean told him that Attorney General John Mitchell and his deputy “are anxious to discreetly find out” whether Kopechne had visited Greece in August 1968.

The file said FBI Director J. Edgar Hoover agreed to look into it but doesn’t explain what, if anything, was discovered.

* A Dec. 17, 1969, memo said Ted Kennedy’s special assistant asked for FBI help after the senator was sent a blackmail letter and “three obscene Polaroid photographs which obviously had been doctored.”

The photos placed the heads of Kennedy, his brother Robert, Kopechne, Jackie Kennedy and Coretta Scott King on the bodies of other people. The letter demanded $100,000.